If he is still eating, it's not very likely that he's got internal parasites. A lot of the time when a betta spends too much time in a cup, bits of slime coat come off and the feces breaks down a lot and can look like slimy stringy bits. The best way to figure out if he has internal parasites is to put him in a bare bottomed container--I use rubbermaid/sterilite plastic storage bins. This way you can clearly see what his feces looks like.

Generally, when a fish has internal parasites they have a distended belly, they are less active or lethargic, they don't want to eat, and their poop is abnormal. If you notice more symptoms like these, I highly suggest a medication with the ingredient praziquantel. I have had some success with Jungle Parasite Clear tabs.

You should also consider changing to a higher quality pellet food brand. Hikari leaves much to be desired nutritionally. I recommend OmegaOne Betta Buffet Pellets, Atison's Betta Pellets, Ken's Betta Crumbles, and New Life Spectrum. It's also a great idea to supplement his pellet diet with a few frozen foods such as brine shrimp and blood worms. Remember to always pre-soak any dry food you feed to your betta in a bit of tank water--this helps prevent bloating.

THANK YOU SO MUCH! Whew i was getting very scared. I was thinking the pieces were a little too small. He is very active, and very hungry so Im alot less worried. Ill try and get some higher quality pellets though Hikari has been recomended?

Hikari used to be a lot better than it is now. In the past year there was a formula change--not that many people are aware of it and are still recommending it despite the fact that it is now lower quality than it was.